The majority of art in the use of sulfur-containing coupling agents in rubber involves silanes containing one or more of the following chemical bond types:
S—H (mercapto), S—S (disulfide or polysulfide), or C═S (thiocarbonyl). Mercaptosilanes have offered superior coupling at substantially reduced loadings; however, their high chemical reactivity with organic polymers leads to unacceptably high viscosities during processing and premature curing (scorch). Their undesirability is aggravated by their odor. As a result, other, less reactive coupling agents have been found. Hence, a compromise must be found between coupling and the associated final properties, processability, and required loading levels, which invariably leads to the need to use substantially higher coupling agent loadings than would be required with mercaptosilanes, and often also to the need to deal with less than optimal processing conditions, both of which lead to higher costs.
The prior art discloses acylthioalkyl silanes, such as CH3C(═O)S(CH2)1-3Si(OR)3 (M. G. Voronkov et al. in Inst. Org. Khim., Irkutsk, Russia) and HOC(═O)CH2CH2C(═O)S(CH2)3Si(OC2H5)3 (U.S. Pat. No. 3,922,436 to R. Bell et al.). Takeshita and Sugawara disclosed in Japanese Patent JP 63270751 A2 the use of compounds represented by the general formula CH2═C(CH3)C(═O)S(CH2)1-6Si(OCH3)3 in tire tread compositions; but these compounds are not desirable because the unsaturation α,β to the carbonyl group of the thioester has the undesirable potential to polymerize during the compounding process or during storage.
Prior art by Yves Bomal and Olivier Durel in Australian Patent AU-A-10082/97 discloses the use in rubber of silanes of the structure represented by R1nX3-nSi-(Alk)m(Ar)p—S(C═O)—R where R1 is phenyl or alkyl; X is halogen, alkoxy, cycloalkoxy, acyloxy, or OH; Alk is alkyl; Ar is aryl; R is alkyl, alkenyl, or aryl; n is 0 to 2; and m and p are each 0 or 1, but not both zero. This prior art, however, stipulates that compositions of the structures of Formula (1P) must be used in conjunction with functionalized siloxanes. In addition, the prior art does not disclose or suggest the use of compounds of Formula (1P) as latent mercaptosilane coupling agents, nor does it disclose or suggest the use of these compounds in any way which would give rise to the advantages of using them as a source of latent mercaptosilane.
U.S. Pat. No. 4,519,430 to Ahmad et al. and U.S. Pat. No. 4,184,998 to Shippy et al. disclose the blocking of a mercaptosilane with an isocyanate to form a solid which is added to a tire composition, which mercaptan reacts into the tire during heating, which could happen at any time during processing since this is a thermal mechanism. The purpose of this silane is to avoid the sulfur smell of the mercaptosilane, not to improve the processing of the tire. Moreover, the isocyanate used has toxicity issues when used to make the silane and when released during rubber processing.
U.S. Pat. No. 3,957,718 to Porchet et al. discloses compositions containing silica, phenoplasts or aminoplasts, and silanes, such as xanthates, thioxanthates, and dithiocarbamates; however, the prior art does not disclose or suggest the use of these silanes as latent mercaptosilane coupling agents, nor does it suggest or disclose the advantage of using them as a source of latent mercaptosilane. There remains a need for effective latent coupling agents which exhibit the advantages of mercaptosilanes without exhibiting the disadvantages such as described herein.